Method and material for producing coatings on metal



Patented Sept. 2, 1 952 METHOD: ANDMATERIAL FOR PRODUCING COATINGS N METAL Robert 0. Gibson, Birmingham, Mich; assignor to Parker Rust Proof Company, Detroit, Mich.

No Drawing. ApplicationNovember 22, 1947,

Serial No. 787,621

'9 Claims. (01. 143-017) 7.

I This invention relates to a method and ma! terial for producing phosphate coatings on metals. l l

More-particularly it relates .to a method'of producing l predominately iron phosphate Lcoatingspossessing a combination of .properties heretofore unknown. 1.

Methods for obtaining ferrous phosphate coatings on metals have been known for many years. This is illustrated by United States Patent No. 870,839 issued November 12th, 1907 to Coslett. He used a boilingsolution of phosphoric acid with some iron dissolved therein and into which the articles were immersed and left until the coating formed. The processing was continued until the coating action ceased, as shownby cessation of bubbling, usually requiring; over an hour. The completed coating weighed upwards of 1000 milligrams per square foot of surface treated.

.Later. developments added refinements to Cosletts method. It was found that the presence of other metals such as manganese added to the bathincreased the corrosion resistance somewhat. speed up the coating action, and the presence of. certain oxidizing agents was found not only to-reduce the time for producing-a substantially continuous coating, but also that stage was reached while the coating 'was much thinner. This is illustrated in United States Patent 1,911,726 issued May 30, 1933, to Tanner and Lodeesen. The coatings produced commercially under; that patent thataveraged to have not more than one-sixth the thickness of the average coating produced under the Coslett patent.

lnqall of these developments, solutions of dihydrogen phosphates of. metals whose less ,acid phosphates are insoluble in water have been required. There arecertain properties common to. all of these coating chemicals The dihydrogen phosphates of these metals are decidedly acid, usually developing a pH value of lower than 21in the coatingsolution, depending upon the strength of solution and the temperature; They; allgproduce coatings better and more rapidly when thesolution is fresh, but the solution hydrolyzes on being heated, and in so doing produces less acid phosphates which are thrown down as-a sludge, leaving the solution more acid than when the dihydrogen phosphate is first dis solved. Agitation, such as obtained by spraying,

accelerates hydrolysis with the inevitable produc-f ti on; of free phosphoric acid in the solution. This acid cannot be neutralized without the fo'rma tion of additional sludge byfurtherhydrolysis, and release of more free acid. When metal is first cleaned with strongly acid oralkaline solutions the phosphatecoating tends to; be deposited in the form of large crystals which render the coating unuseablefor certain applications. This condition can be avoided-only by adding extra steps to theprocedure.

Copper in small amounts was found to It is customary with all commercial phosphate factory reaction withthe metal. This is usually done with steamcoils immersed in the solution. The sludge mentioned above gradually forms a crust on the coils so that heat transfer is retarded and the coils must-1 beqremoved and cleaned at frequent-intervals. The sludge also causes difliculties, especially in installations where the solution is sprayed and recirculated, by plugging nozzles and pipes, thus necessitatingadditional maintainence cost.

An object of this invention is the production of an iron phosphate coating without forming more than a negligible amount of sludge.

w Another object is the formation in a short time ofan extremelythin coating which is equivalent to the usual thicker phosphate coatings as apaint base. i

A further object is to provide a method of treatment whereby the pI-I'of the solution automatically remains high, thus avoiding the necessity for continued adjustment.

Other objects will become apparent as the description proceeds. V v

It is known that the dihydrogen phosphates of the alkali metals, including ammonium, do not possess any direct coating ability because their less acid phosphates'are all water soluble. Where alkali metals are mentioned hereinafter the term includes ammonium. They do however produce a smallamount of free phosphoric acid in aquephates of zinc, iron and manganese for example.

This is important both from the standpoint of economy of operation. and in avoiding certain production difliculties. There is much less metal dissolved during treatment and the formation of rust on the coated metal from steam during line stoppage is substantially eliminated. This has always been a source of trouble with solutions which operate in a low pH range.

. The low acidityis apparently very effective in precipitating the dissolved iron immediately as coating .since practically no dissolved iron is found in the solution. Furthermorethe original pH 'ofthe bath actually rises somewhat in the I g initial'stages of userather than becoming lower,

' with"'ident 'cal 'conditio 3 such conditions isjfor the coatingto exhibit sparklefi wliich is caused by unusually coarse crystals. vention since the resulting coatings are fine grained.

The invention resides in the discovery that mm oxidizing agent is added to solution-oidihydrogen This is minimized by the present in.-

of paint adhesion; Cmparab1e= results, as indicated by salt spray tests-,ihave been obtained with coatings ranging from 65 down to 22 milligrams per square foot of surface treated. The latter" phosphate of an alkali metal, a coatinglofiinicreased thickness is formed, exactly the opposite of the effect of similar agents with phosphatesof coating metals, and some alterationoin the physical or chemical structure of the coating occurs which greatly augments its value as a paint baseto' the pointwhere it v is equal to l the much heavier coating obt 'ained with the staind'ard phos mate-process customarily used? Moreover these resultsare accomplished" wit-Ha coating weight" which is unusually. lows This is hi'ghly -iinportant' in maintaining maximum 19311151118116}? andal's'o where it 'is desired to painta coated sheet and thereafter form it to the desired sha'pe. Heavier:-

- val'ue'iscomparable to that obtained in the abwith a concentration of sodium dihydrogemphpsphateof il 4 grams per, liter. and; .5 C OQLshQuld prevail; Greateramountszofitphosphategthanjth-is coatings, for example weighing'over 1 00 milli gramsper square f-oo't -of surface treat'ed ','are' totau unsuitable'for some such formingz-op'era tions. Initial adhesion of certain types- -o flpaint has been found to be'=-s'uperior to that obtained withlieaviercoatingst' Investigation has shown that: there are certain limitations th'atmust beob'serve'd i f maximum benefit of: the invention is: to be obtaineditpH- of the operating solution should nettle-sub stantially lower than that normally-.obtainedi by: the dihydrogen phosphate of the a'lkal i metal, or 4 .2. Increased "acidityi over 'thattamountg iv'en gives inferiorresult's in paint' a'dhesion. Theitotal' concentration of the" bathv both in resp'e'ctto the phosphate and oxi'di zi'ngi'a'gent have aibearting on ultimate results obtained. Differentwoxi'dizing agents are effectiveindifiere'nt amounts, but in each cas'e' a minim'u niis required to cause the necessary change coating structure' in-orden to obtain best results- 'F-or example if: cmbmte is by spraying'for a period of one minute waizen used there appear's a rather'sharp improvement I in re's'ults-when .5 ClOEtis present The amount of the oxidizing radical is given herein whenever nototherwise specified, since itis of the'amount or this radical which is essential regardl'ess of the atomie weight of the baseunited therewithi- Higher" amounts maybe usedup to at least *4 ands om'eti-mes even more,{with= som added improvementbutat' least ;5 %=-shou-l'd be used; a

V Nitrite is more; effective 5 than chlorate that only a tenth'as much is required for equivalent results. Bromate maybe usedwith good results with concentrations ranging from-,2 to" 2. 0 BrOa:

Sulfite is likewisefeifective'. ;A good worki'ngrange agents used in -plios'pha'te coating operations with phosphates oi coating chemicals; is substantially without effect the present; instance; probably due to the high fP'erniangan-a'te' also "5' inr xcept for thei'r prs once.- Uns'uitable agerits' immense amounts of suitable agent's doi-not produce this result-f mates-other than-those of th'e alkalimletals m In spite of this distinction it does mangoes:

sarily follow that-there is a direct-relationship between the amount of coating-and thee-Xcellence 'i'ntadhesi'om I t r it v A dilute-solution of chromic a'cid or diciiro canbfe usedibuttheretappe'arszto benoiadvanta-gez;

in a higher concentration, andconsuinptlomis: increased because: of: an': 'mcneaseitamonmi; of chemicals carried outlomthe worln.- Usually the; pI-Lva'luewill be substantially higher: than lfii as, a result of'reaction with the. metal and: thez plres'e. ence of 'salts in ordinary water: which reacttwitht the phosphate reduce: the acidity: somewhat;;.: I

In a commercial installation solution: was; prepared by dissolvingrrin about, 9.09.; gallo'nsrz ixwaterg-ll'3 pounds sodium dillydrogencphosphate: and sufficient: sodiumzctrloratezoto; furnish. C103:- The I solution. wasmeateda torlfioff'Fsandi emulsion cleaned steelparts-rwerercoated therein;

rinsedthen' finally rinsed in a dilute. hot mixture of phosphoric and chromic .acid': Afiter :produej tion had continued for ashontztime:thetpltofithe solution was found to have reached; a: value; as

high as'5.2. 1 f: v Y a 5,11 2

Numerous: line stoppages: wherewthe. coated metal was subjected tohoit vapors foriext'ended period's caused no objectionable amount. ofifrust ingin any case and" in} most cases? evidence: 0t attack was entirely absent although the coating weightseldom exceeded milligrams per square P footof surface; treated. Y I I The sludge which accumulated amounted-ta only about one-tenth pound for a eachthousand square feet ofsurface treated;- 'I en times that amountimayJoe-produced withcenveional phos phateztreatnients; Atndtime was itl -nec essary tocle'an' the heatingcoils'or spray ne'zzles "Chemical consu'inption amounted to' lil per-tho'usandsquare feet or aboutone-halftliht ordinarily obtained Where the majorityxof the coating is derived-fromchem-icals- 'furnished dissolved phosphates of 'z i'nc; manganeseyete.

' The-acidified fina'irinse aids ngprovioi i th maximum resultsv in pai-nt adhesion, "especially where the-{oxidizing agent is neaf'ts lower-effectiVeli'mit; andforestalls possible i'ciilties 'fiioin blistering; which may becaused'by the 'reteiiti-on of unconvertedhard wate'rsaltsonthe coated surface: 'Where-as muchas-'4%"chlerat'e was cluded' in the solution; the final ra ses-stas s phoric and chromic acid made little difleijeriee be-used instead or thevmixture of phosphoric and The quality of the coating produced by r A material for preparing and replenishing the solution may be made by mixing the correct proportions of phosphate and oxidizing agent. This may be furnished as a dry combination or as a concentrated aqueous solution. A suitable ratio of phosphate and sodium chlorate to maintain the phospate substantially constant and the chlorate at approximately .5% is Pounds Sodium dihydrogen phosphate or equivalent 1.36 Sodium chlorate .53

Other compatible phosphates of alkali metals and oxidizing agents may be mixed. phosphate and sodium chlorate would form an explosive mixture and so are incompatible for a dry mix, although they may be used together in solution.

To recapitulate, my invention makes use of the dihydrogen phosphates of the alkali metals at pH values not lower than approximately 4.2 and not higher than 6.

Certain oxidizing agents, equivalent to .5% to 4% chlorate, added to the solution have been found to cause an unforeseeable alteration in the coatings over that obtained by the phosphate alone which makes them exceptionably valuable as a paint base. Among these agents are chlorate, nitrite, bromate, and sulfite. characterized by their ability to increase the coating weight over that obtained by the phosphate alone, as well as modifying the quality of the coating, whereas oxidizing agents not suitable, such as nitrate and permanganate, do not exhibit this ability.

Specific examples of solutions have been given to illustrate the invention, it being understood that variations may be made as indicated without departing from the scope of the invention.

The phosphates of different alkali metals may be used, and the base of the chlorate or other oxidizing radical is immaterial provided that it is not of a kind that will interfere with the coating action. No danger of this is present when only alkali metals are present in the oxidizing agent.

In the claims, as well as in the foregoing specification, alkali metal is used in its broad sense which includes ammonium.

What I claim is:

1. The process of producing a phosphate coat-= ing on a ferrous surface, which consists in applying to the surface a solution consisting essentiallyof water, acid phosphate of an alkali metal and. an oxidizing agent of the group consisting of nitrites, sulphites, chlorates and bromates of the alkali metals, the oxidizing agent being present in an amount having an eifect on the quality and rate of coating on the said ferrous surface substantially equal to that produced by .5% to 4% chlorate, and the solution having a pH of from 4.2 to 6, and continuing the application until a coating results. v

2. The process of producing a phosphate coating on a ferrous surface, which consists in applying to the surface a solution consisting essentially of water, acid phosphate of an alkali metal and a chlorate of an alkali metal, the chlorate being present in an amount from about .5% to about 4% and the solution having a pH of from. 4.2 to 6, and continuing the application until a coating results.

3. The process of treating a ferrous surface to receive a siccative coat which consists in applying to the surface a solution consisting essentially of water, acid phosphate of an alkali metal and Ammonium 1 These are all an oxidizing agent selected from the group consisting of nitrites, sulfites, chlorates and bromates of the alkali metals, said oxidizing agent being present in an amount having an effect on the quality and the rate of coating on the said forrous surface substantially equal to that produced by about .5% to about 4% chlorate ion, said solution having a pH from 4.2 to 6.0; continuing the application until a coating results and thereafter rinsing the said coating in a dilute solution of chromic acid.

4. A process of treating a ferrous surface to receive a siccative coat which consists in applying to the surface a solution consisting essentially of water, acid phosphate of an alkali metal and a chlorate in an amount from about .5% to about 4%, said solution having a pH from 4.2 to- 6.0, continuing the application until a coating results and thereafter rinsing the said coating in a dilute solution of chromic acid.

5. A solution for producing a phosphate coating on a ferrous surface which consists essentially of water, acid phosphate of an alkali metal and an oxidizing agent selected from the group consisting of chlorates, bromates, nitrites and sul lites, said chlorate, when present, being present in an amount from .5% to about 4%; said bromate, when present, being present in an amount from 2% to 2.0%; said nitrite, when present, being present in an amount from 05% to about 4%; said sulfite, when present, being present in an amount from .2 to .5%, said solution having a pH from 4.2 to 6.0.

6. A solution for producing a phosphate coating on a ferrous surface which consists essentially of water, acid phosphate of an alkali metal and a chlorate of an alkali metal in an amount from about .5% to about 4%, Said solution having a DH from 4.2 to 6.0.

'7. A solution for producing a phosphate coating on a ferrous surface which consists essentially of water, acid phosphate of an alkali metal and a nitrite of an alkali metal in an amount from about .05% to about .4%, said solution having a pH from 4.2 to 6.0.

8. A solution for producing a phosphate coating on a ferrous surface which consists essentially of water, acid phosphate of an alkali metal and a sulfite of an alkali metal in an amount from .2% to .5 said solution having a pH from 4.2 to 6.0.

9. A solution for producing a phosphate coating on a ferrous surface which consists essentially of water, acid phosphate of an alkali. metal and a bromate of an alkali metal in an amount from about 2% to 2.0%, said solution having a pH from 4.2 to 6.0.

ROBERT C. GIBSON.

REFERENGES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,320,734 Chadwick et al Nov. 4, 1919 2,132,883 Romig Oct. 11, 1938 2,268,323 Martin et al Dec. 30, 1941 2,302,516 Tanner et a1 Nov. 17, 1942 2,302,643 Thompson Nov. 17, 1942 7 2,304,299 Boyle et al. Dec. 8, 1942 2,312,855 Thompson Mar. 2, 1943 2,403,426 Douty et al July 2, 1946 2,477,841 Ward Aug. 2, 1949 2,479,423 Snyder Aug. 16, 1949 

1. THE PROCESS OF PRODUCING A PHOSPHATE COATING ON A FERROUS SURFACE, WHICH CONSISTS IN APPLYING TO THE SURFACE A SOLUTION CONSISTING ESSENTIALLY OF WATER, ACID PHOSPHATE OF AN ALKALI METAL AND AN OXIDIZING AGENT OF THE GROUP CONSISTING OF NITRITES, SULPHITES, CHLORATES AND BROMATES OF THE ALKALI METALS, THE OXIDIZING AGENT BEING PRESENT IN AN AMOUNT HAVING AN EFFECT ON THE QUANLITY AND RATE OF COATING ON THE SAID FERROUS SURFACE SUBSTANTIALLY EQUAL TO THAT PRODUCED BY .5% TO 4% CHLORATE, AND THE SOLUTION HAVING A PH OF FROM 4.2 TO 6, AND CONTINUING THE APPLICATION UNTIL A COATING RESULTS. 